This invention relates generally to thermal management systems and more particularly to thermal management systems adapted for use in electronics systems such as, for example, portable computers.
As is known in the art, with the growth of personal computers, there has been increasing demand for portable computers, such as notebook, laptop and palmtop digital computers. The first portable computers, known as "lugables", were AC powered and utilized the same power supply, and printed circuit board technology, as their desk top cousins. Over time, advancements in power generation (i.e., the use of rechargeable batteries), and power management (i.e., extending the time interval between battery charges) have further reduced their size thereby further increasing their usefulness and demand. However, removal of heat generated by electronic components, or other heat generating source, within a small notebook computer is significantly more difficult than removal of heat from a desk top computer. Because of the relatively large space available in a desk top computer, heat transfer may be managed through the use of fans and principles of thermal convection cooling. Notebook computers, on the other hand, because of the more limited space, are not suitable candidates for cooling fan, or forced convection type thermal management systems.
Techniques and devices which have been suggested to manage heat in notebook computers are discussed in an article entitled "Hot Problem? Cool Solution!", by Gary Kuzmin, published in EDN Products Edition, Feb. 21, 1994. One system described therein is referred to as the Oasis.TM. fluid cooler. Such system operates in similar fashion to a heat pipe; however, it includes flexible, plastic materials forming an enclosure, allowing the pressure inside the enclosure to equalize with the pressure outside the enclosure. The operating temperature remains constant for wide heat load variations because it is a boiling heat transfer system. More particularly, a liquid boils in the enclosure attached to the component to be cooled. The resulting vapor condenses back to a liquid as it releases its heat to a condensing wall of the enclosure at the other end. The liquid then flows back to the evaporator, completing one of a series of constantly repeating cycles. While such system is adequate in some applications, it is not able to operate in all orientations. More particularly, the condensing wall (i.e., heat sink) must be at a higher elevation than the component to be cooled (i.e, the heat source). In other applications, however, it may be desirable to remove the orientation dependence of the system, as well as reduce the complexity and cost of such system.
In another suggested system, a capillary arrangement is used to transfer the condensed vapor back to the heat sink. More particularly, an enclosure is provided with portions thereof comprising a thin flexible, thermally conductive sheet and a flexible, typically plastic, membrane. The flexible membrane has peripheral ends attached to a first surface of the thermally conductive sheet. One end of the opposite surface of the thermally conductive sheet is adapted for mounting to a heat source. The other end of the aforementioned opposite surface of the thermally conductive sheet is adapted for coupling to a heat sink. A fibrous, liquid transporting material is disposed within the enclosure. Opposite end portions of the fibrous material are disposed adjacent the heat source and the heat sink, respectively. A small amount of liquid is disposed on the end of the material adjacent the heat source. Heat emanating from the heat source is received by the thin thermally conductive sheet and transferred to the liquid thereby converting the liquid to a vapor. The vapor passes within, and through, the enclosure for transportation to the end of the fibrous material disposed adjacent the heat sink. The transported vapor is condensed to the liquid as the vapor transfers its heat through the thin thermally conductive sheet to the heat sink. The condensed liquid is then transported by capillary action provided by the fibrous material back to the heat source to complete one of a series of continuously repeating heat transfer cycles.